1. Field of the Invention
The present invention relates to a vehicle wheel information supply device for supplying wheel information relating to a state of a wheel of a vehicle, which device includes a transmitter disposed on the wheel and a receiver disposed on the body of the vehicle. The present invention is also concerned with a wheel tire abnormality indicating device which includes such a wheel information supply device, and an indicator operated in response to an output of the vehicle wheel information supply device, to indicate an abnormality of a tire of the vehicle wheel.
2. Discussion of the Related Art
JP-U-5-13802 discloses an example of such kind of vehicle wheel information supply device in the form of a tire air pressure information supply device adapted to supply tire air pressure information indicating whether an air pressure in a tire of a wheel of an automotive vehicle is normal or not. This tire air pressure information supply device includes an air pressure detector disposed on the vehicle wheel to detect the air pressure of the wheel tire, an air pressure information transmitter also disposed on the vehicle wheel to transmit the tire air pressure information, and an air pressure information receiver disposed on the vehicle body to receive the tire air pressure information transmitted by the air pressure information transmitter. Based on an output of the air pressure detector, the air pressure information transmitter intermittently transmits a pulse signal of a predetermined pattern when the output indicates that the tire air pressure is normal, and continuously transmits a pulse signal of another predetermined pattern when the output indicates that the tire air pressure is abnormal. Thus, the air pressure information transmitter in the tire air pressure information supply device described above is adapted to transmit a pulse signal in different patterns where the tire air pressure is normal and where it is abnormal.
The air pressure information receiver disposed on the vehicle body includes a receiving portion including an antenna. The air pressure information transmitter disposed on the vehicle wheel is rotated with the vehicle wheel, so that a distance between this transmitter and the receiving portion of the air pressure information receiver periodically changes, causing a periodic change in the amplitude or intensity of the signal received by the receiver. When the distance is relatively large, the signal-to-noise ratio (SN ratio) is relatively low, and the receiver may not receive the signal whose intensity is high enough to correctly represent the air pressure information. Where the time period required for the transmitter to transmit the air pressure information is relatively long, the probability of incorrect reception of the information is lower due to an influence of the noise on a part of the information than where the time period is relatively short. Accordingly, the probability of correct reception of the entire information by the receiver ("reception ratio") is lowered with an increase in the time period of the transmission. In the tire air pressure information supply device disclosed in the above-identified publication, the time period of the transmission where the tire air pressure is normal is the same as that where the tire air pressure is abnormal. The reception ratio is lower when the rotating speed of the vehicle wheel is relatively high than when the rotating speed is relatively low, provided the time period of the transmission and the noise level are constant. This is because the ratio of the time period required to transmit a series of the air pressure information to the time period required for one revolution of the wheel increases with an increase in the rotating speed of the wheel.